What Size Backpack Do I Need? Liters and Inches Explained

What size backpack do I need? The first time I answered that question, I got it wrong, and I spent three weeks in Southeast Asia with a 70-liter pack. It was my first big trip, and I had reasoned that bigger meant more freedom. In reality, I carried 33 pounds (15 kilos) for three weeks, of which I used maybe half, and I spent my time digging for things at the bottom.

On the next trip, I took a 45-liter pack. I did not miss anything I had left behind, and I walked much better. That is when I understood something simple. The space you have always ends up filled. The bag does not adapt to your stuff, your stuff adapts to the bag.

So here are my benchmarks for backpack sizes in liters, use by use, drawn mostly from my own bad math, with the inches that product pages and airlines use wherever they help.

• Why the size of your backpack matters
• Backpack sizes in liters and what each one is for
• What size backpack do I need for hiking, backpacking, travel or the city?
• What to check beyond the liters
• My backpack size chart in liters and cubic inches
• Practical tips for filling your backpack
• The questions I get asked, from liters to inches

Why the size of your backpack matters

Carrying comfort comes first

A bag that is too big is not just bulky. It changes where the weight sits. On my 70 liters, the contents floated and slumped down and backward with every step, and I spent my time tightening the compression straps. A bag that is eighty percent full carries much better than a big bag that is half empty.

Matching the volume to your body

This point had escaped me. The capacity in liters says nothing about the length of the back. A 50-liter pack can come in several back lengths, and that length decides whether the hip belt lands on your hips or on your ribs. I am 5 ft 5 in (1.65 m), and for a long time I carried bags whose belt sat too high, telling myself it was normal.

How to measure your torso length

Packs are sized on your torso length, not on your height. With someone to help you, measure in inches from the bony bump at the base of your neck, where it meets your shoulders, down to the point on your spine level with the top of your hip bones. Then compare that figure with the brand’s size chart. Some larger packs have an adjustable suspension, so you can set this length yourself, and some brands also give a range of hip sizes for the belt.

What a bag that is too heavy costs you

Over my three weeks in Asia, I carried 33 pounds. The real cost was not the tiredness in the evening. It was that I chose my routes around the bag. I took cabs where I would have walked. A bag that is too big makes the trip smaller.

Backpack sizes in liters and what each one is for

Small backpacks, 10 to 30 liters

The city, the office, a day out. At 20 liters you can fit a laptop, a charger, a water bottle and lunch, which covers a normal day. Below 15 liters with a laptop, the laptop takes up everything. I have tried it.

Medium backpacks, 30 to 50 liters

My favorite size, by far. Two to four days staying in mountain huts, or a light trip of several weeks if you are willing to do laundry along the way. It is also the upper limit of what you can still carry comfortably for a whole day.

Large backpacks, 50 to 80 liters

Full self-sufficiency, with a tent, a stove and food for several days. It is the size you need when you really are going self-supported, and the one not to take when you are not. My 70 liters belonged to this category, and I had bought it for a trip staying in hostels.

Over 80 liters

Expeditions, winter, technical gear. Unless you have a very specific need, it is a size that concerns almost nobody, and its empty weight is already considerable.

Why two 35-liter backpacks do not always hold the same

Brands do not all measure liters the same way. Some count only the main compartment, others include every pocket, side and hip belt pockets among them, so two packs sold as 35 liters can hold different amounts. The liters also say nothing about shape. A pack with a generous stated volume can still be too short for a laptop, which is why the dimensions in inches matter as much as the capacity.

What size backpack do I need for hiking, backpacking, travel or the city?

For a day hike

20 to 30 liters. Water, snacks, a warm layer, a rain jacket and a first aid kit. I have seen a lot of people head out for the day with a half-empty 40 liters, and the bag sags out of shape and sways with every step.

For a multi-day backpacking trip

40 to 50 liters if you sleep in huts, 55 to 65 liters self-supported with a tent. The difference comes almost entirely from the tent, the sleeping pad and the food. In winter or in cold weather, allow a little more, because the extra layers and a warmer sleeping bag take up room.

For travel with a backpack

40 to 50 liters, and I say so with the conviction of someone who has tried 70. A 45-liter pack often goes in the cabin on long-haul flights and saves you the wait at baggage claim. What I gained by going smaller far outweighs what I lost. See also our travel backpack collection.

For the city

15 to 25 liters. Above all, check that the laptop compartment is padded and raised off the bottom, and measure your laptop itself with a tape measure rather than trusting the screen size in inches. It is the most common buying mistake. The models in our laptop backpack collection cover this size. For the office, see also how to choose a work backpack for women.

What to check beyond the liters

Shoulder straps, other straps and the hip belt

Above 30 liters, the hip belt is not an accessory. It is the part that should carry 60 to 70 percent of the weight. On my first big pack, it was thin and unpadded, and everything fell back onto my shoulders. It took me a long time to understand that the problem was not the weight but the belt.

When you try a pack on, the belt should sit on the top of your hip bones, not around your waist, and the shoulder straps should hold the pack close without digging in. On larger packs, the short load lifter straps at the top of the shoulder straps pull the top of the bag in toward you, so it does not pull back on your neck and shoulders.

Waterproofing and materials

Nylon resists abrasion better than polyester of the same denier. Few bags are truly waterproof, and a rain cover is not enough, because water runs down your back and gets in at the top. Since one memorable stormy evening, I add dry bags inside for my sleeping bag and dry clothes. Our waterproof backpacks collection has the models designed for that.

Access and storage

It is the criterion I rank highest today. A bag that only opens from the top makes you empty everything to reach the bottom. A suitcase-style opening changes everything when you travel, and I would not head out without one again. Our carry-on backpack that opens like a suitcase is built on this principle.

Body shape

Women’s models have a shorter back and curved shoulder straps, narrower at the base. It is not cosmetic. On the standard version of the same bag, the straps kept sliding toward the outside of my shoulders. Our backpacks for women collection includes these adjustments.

My backpack size chart in liters and cubic inches

Some brands still give capacity in cubic inches. One liter is 61.02 cubic inches, which is how the second column is worked out.

One figure I now check every time, and that I used to ignore completely, is the empty weight. Two 50-liter bags can differ by 1.8 pounds (800 grams), the weight of a whole water bottle.

Practical tips for filling your backpack

Spreading the weight

Heavy items in the middle, against your back, at shoulder blade height. Bulky, light items at the bottom. Quick access at the top. I used to put heavy things at the bottom because it seemed logical, and that is exactly the opposite of what you should do. I go through the packing order in my guide on how to pack a backpack for hiking.

Roll rather than fold

Tightly rolled clothes take up much less room and wrinkle less. Packing cubes have saved me a lot of time when traveling. Instead of rummaging, I take out the right cube.

Using the compression straps

They are not there to crush the bag, but to hold the contents tight against your back. A compressed bag carries like a smaller one. That is what I was doing wrong on my 70 liters, and it is why it swayed.

Traveling light, in practice

• One change of clothes, not three. You do laundry along the way.
• Repack food out of its original packaging.
• Fill your water bottle along the way rather than heading out with two liters (about half a gallon). A liter of water weighs about 2.2 pounds (1 kg).
• Weigh the bag before you leave. It was by putting mine on the bathroom scale that I discovered my 33 pounds.

For uses where the volume changes from one day to the next, the multi-purpose expandable travel backpack, which expands from 9.1 to 12.2 inches (23 to 31 cm) deep, saves you having to choose between two sizes.

The questions I get asked, from liters to inches

How do I know what size backpack to get?

Start from the use and the number of days, then check the fit. As above, that is 15 to 25 liters for the city, 20 to 30 for a day hike, 40 to 50 for travel or a hut trek, and 55 to 65 with a tent. Then make sure the back length matches your torso and that you can carry the bag full. If you hesitate between two sizes, remember that the space you have always ends up filled.

Can I take a 40L backpack as a carry-on?

Often, yes, but compare the dimensions too, because an airline’s limit is given in inches more often than in liters. My rule is to stay under about 15.7 x 7.9 x 21.7 inches (40 x 20 x 55 cm), which is 35 to 45 liters depending on the shape. 22 x 14 x 9 inches is the carry-on limit as stated by United, American, Delta, Alaska and JetBlue, and Southwest and Frontier allow 24 x 16 x 10 inches. The 22 x 14 x 9 box is narrower than my rule (14 inches against 15.7) but deeper, so check the width. Check your own airline’s dimensions, because they vary a lot on budget airlines. Our guide to carry-on bag size by airline lists each one. For checked baggage, strap down everything that dangles, otherwise the straps get caught on the conveyor belts. See our carry-on backpacks.

Is 17 inches big for a backpack?

It depends on what the 17 inches measure. If it is the height of the bag, it sits between two of our hiking packs. For comparison, our ultralight 10L hiking backpack measures 9.1 x 3.9 x 15.7 inches (23 x 10 x 40 cm) for 10 liters, and our large hiking backpack with hip belt measures 18.5 x 12.2 x 8.3 inches (47 x 31 x 21 cm) for 20 to 35 liters. If it is the laptop the bag takes, it says even less about the volume. Our ergonomic carry-on 17 inch laptop backpack takes a laptop up to 17 inches in 15.7 x 11.8 x 7.9 inches (40 x 30 x 20 cm) and 30 liters.

Is 16 inches a full size backpack?

Sixteen inches (about 41 cm) alone does not tell you how much a bag holds, whether it is the height or the laptop size. Two of our bags that take a 16-inch laptop show why. Our men’s faux leather backpack for 16 inch laptop holds under 20 liters at 11.4 x 5.9 x 16.9 inches (29 x 15 x 43 cm), and our trekking backpack 36 to 55 L adjusts from 36 to 55 liters. For a normal day of school or work, 20 liters already covers it, as I said above.

What size hiking backpack for women?

The same liters as for everyone, but a shorter back length and curved shoulder straps. That setting is what matters, not the volume. A 5 ft 3 in (1.60 m) hiker will be better off with a 45-liter pack with a short back than with a 40-liter one with a standard back.

To compare volumes and carrying systems, have a look at our hiking backpacks.

How much weight can I carry in a backpack?

Around 20 percent of your body weight for hiking. For travel, I aim for 22 to 26 pounds (10 to 12 kg) all in, because you carry the bag up train station stairs and not only along a trail. My 33 pounds in Asia were well above what I can carry comfortably.

Backpack or suitcase for travel?

The backpack wins as soon as there are stairs, cobblestones, public transit or walking. The suitcase wins for a stay in a single hotel with a cab transfer. I have both, and I take the backpack nine times out of ten.

What size backpack do I need? What my 70 liters taught me

Go with the volume of your real needs, not of your imagined ones. A well-filled 45 liters will take you further than a half-full 70 liters, literally.

And weigh your bag before you leave. It is the most useful habit of all, and nobody does it.